This invention relates to an adjustable deflection yoke mounting for cathode ray tubes and, more particularly, to yoke mounting for in-line type multiple electron beam color picture tubes.
In color television receivers, the common type of image reproducing device is a cathode ray tube having three electron beams to selectively excite groups of phosphors capable of respectively producing light of three different colors, such as red, green and blue. Such a tube has a luminescence screen and in a common type of delta gun receiver this screen is made up of a plurality of triads of phosphor dots which are excited by respective electron beams projected through suitable placed apertures in a shadow mask located in back of the screen. The beams strike their respective phosphor dots by approaching them through different directions. One commonly employed arrangement of the electron guns, and hence the electron beams emanating therefrom, is with the blue gun located directly above the longitudinal tube axis and with the red and green guns located below this axis and respectively at opposite sides of the axis horizontally. Because of the beam arrangement relative to the longitudinal axis of the picture tube and the particular character of the deflection yoke field, there is a tendency for one raster (for example the blue raster) to be of a different width than the other two when full screen static and dynamic convergence is achieved. Misconvergence takes place at the sides of the picture when operating such tubes with large deflection angles. It was found that tilting the rear end of the yoke in the vertical plane only could help in this registration of the rasters. (See, for example Obert et al., U.S. Pat. No. 3,302,050 and Ryder et al., U.S. Pat. No. 3,582,848).
Color television picture tubes utilizing three horizontal in-line electron beams have been used to replace the delta gun tubes to simplify the apparatus required to maintain convergence. In the in-line picture tubes, the phosphor elements are in vertical strips of phosphor material and the apertures are elongated slits also extending in the vertical direction. Such vertical line phosphor element structures eliminates vertical registration problems. However, if the beams are not perfectly aligned with the center of the yoke magnetic field, the beams will not be converged on the viewing screen. In practice, the beams are not properly aligned as a rule and steps must be taken in order to align the beams with the magnetic field. This type of convergence correction in in-line type tubes is discussed in Barbin U.S. Pat. No. 3,789,258. In the Barbin patent, transverse movement of the entire yoke relative to the tube axis is taught as a means for achieving this form of convergence without requiring dynamic convergence correction apparatus. An example of apparatus for accomplishing this is described for example by Shrader in U.S. Pat. No. 3,786,185. Shrader also suggests that convergence may be achieved by generally tilting the yoke relative to the axis of the tube. No specific pivot point of tilt is discussed. In U.S. Pat. No. 3,921,110 of Ishii et al., it is suggested that the end of the yoke nearest the screen (forward end) be moved to adjust for this convergence. The pivot point in this arrangement would therefore be at or near the rear end of the yoke. According to the prior art, the yoke cannot be placed as far forward as it possibly could in order to allow space at the forward or screen end of the yoke to be translated or tilted generally in the X and Y planes (vertical and horizontal). Also, in order to make the purity adjustment, the deflection yoke must be able to be slid forward and back along the neck. This range of adjustment for purity is limited on the back end or end away from the screen by the point at which the beam strikes the neck of the tube and on the forward end (screen end) by the flare of the tube. In the past, the in-line type of tubes required relatively short yokes in order to provide the space for the purity adjustment and convergence adjustment. Short yokes, however, are less efficient and use more energy and therefore this arrangement is contrary to producing a low energy consuming television receiver.